This invention relates to microfluidic instruments for the measurement of total organic carbon (TOC) in water. More specifically the invention relates to rapid, real-time measurement of TOC.
Highly purified water, often in large quantities, is a requirement for a variety of industries. The semiconductor industry, for example, has a particular need for highly purified water for cleaning or as a solvent in the production of integrated circuits. Pharmaceutical and chemical manufacture has a similar requirement for highly purified water. The presence of organic carbon compounds, even in trace amounts, in water used for manufacturing can be deleterious to the quality or purity of products made and the efficiency of manufacturing processes.
Coupled to the requirement for highly purified water is a requirement for accurate, reproducible assessment of water purity. Detection and quantitation of trace contaminants in water can be used to test process water quality, to validate water purification systems and to avoid introduction of contaminated water into process streams or reactions.
The measurement of total organic carbon (TOC) concentration is also used to assess contamination of potable water, municipal water supplies and in industrial and municipal effluents and waste waters.
The organic carbon content of water can be determined by methods which initially oxidize the organic carbon in a sample to CO2 and then determine the amount of CO2 generated in the sample. Organics in water samples can be oxidized in a number of ways by combustion, by use of chemical oxidation agents and/or by UV radiation. In total organic carbon (TOC) analysis, CO2 can be quantitated using infrared (IR) absorption techniques, conversion to methane followed by flame ionization or by measurement of sample conductivity. See, e.g., S. J. Poirier and J. H. Wood (1978) xe2x80x9cA New Approach to the Measurement of Organic Carbon,xe2x80x9d American Laboratory, December:79-89, for an overview of such techniques.
Conventional TOC measurements have several significant limitations. Current devices are relatively slow, taking 120 seconds or more for each measurement. These devices usually require relatively large sample volumes and may require complex sample handling procedures. Current TOC instrumentation is often bulky and unsuitable for portable, on-line applications.
U.S. Pat. No. 3,958,941 of Regan describes a TOC measurement device which employs UV radiation to oxidize organic carbon species in a sample to CO2. The CO2 generated is then transported into pure deionized water and the change in conductivity of the deionized water due to added CO2 is measured. In the Regan method, the sample is mixed with air prior to irradiation to facilitate oxidation of organics. The CO2 generated by oxidation in the sample is transported into the deionized water by an air-stripping system. The method is not sufficiently rapid to allow continuous real-time measurement and requires relatively large sample sizes for accurate measurement. Other methods reported for measurement of TOC in water using conductivity measurements include U.S. Pat. No. 3,224,837 of Moyat, U.S. Pat. No. 4,293,522 of Winkler and more recently U.S. Pat. Nos. 4,626,413, 4,666,860, 4,868,127 of Blades and Godec, and PCT applications WO 94/35498 (Sievers Instruments) and WO 96/01999 (Millepore).
U.S. Pat. No. 4,277,438 of Ejzak reports a TOC measurement device in which air and an oxidizing agent are added to the sample prior to UV irradiation and in which the CO2 generated from oxidation of organics is passed into a gas stream for IR analysis. U.S. Pat. No. 4,619,902 of Bernard also relates to a TOC measurement device using IR detection for CO2.
The Blades and Godec patents supra all relate to a device and method for conductivity measurement of TOC in water using a single sample cell having two electrodes which can be exposed to UV radiation. Background conductivity is measured in the sample before exposure to UV light. When the UV light is switched on, conductivity is measured as a function of time and the second time derivative of the conductivity signal is measured to determine when oxidation is complete. Computer methods are applied to separate out signal due to background conductivity and obtain a measurement of TOC. Complete oxidation of organics in the sample is said to require variously 1-5 min. or 1-20 min. U.S. Pat. No. 4,666,860 further reports a method for prediction of TOC in a given sample based on measurement of conductivity at times before complete oxidation of organics has been achieved. This method requires initial calibration of the device using conductivity measurements over time for complete oxidation of a similar sample. It appears that this method is not useful for measurements where TOC may vary significantly from sample to sample. U.S. Pat. No. 4,868,127 further reports the use of a thin layer of TiO2 formed on the titanium electrodes as an oxidation catalyst and the use of electrophoresis to accelerate reaction.
WO 96/01999 reports methods for rapid determination of TOC in water samples using conductivity measurements. TOC determinations in these methods are, however, predictions based on extrapolation from conductivity measurements at partial oxidation of the organics in a given sample.
Microfluidic devices have been employed for liquid phase analytical applications. U.S. Pat. No. 5,637,469 of Wilding et al. discloses microfabricated devices having a mesoscale flow system for the detection of analytes. Analyte detection is based on the binding of analyte to binding moieties within microfabricated channels in the flow system. Microfluidic devices do not appear to have been employed for the measurement of TOC in water samples.
The present invention represents a significant improvement over the prior art for measurement of TOC of water samples. The microfluidic TOC analysis device of this invention provides rapid measurement, not prediction, of TOC by measurement of conductance in water samples in which the organic carbon is substantially oxidized to CO2 in times less than about 30 sec. The devices of the present invention provide accurate and reliable TOC measurements requiring relatively small sample volumes (30 xcexcL or less). The use of small sample size increases sampling speed and decreases the time between measurements. The relatively small sample cells of this invention can be configured in a compact device suitable for portable instruments. The relatively small size of the sample cells and attendant detectors facilitate use of the TOC measurement devices of this invention in on-line applications.
This invention provides devices and methods for rapid real-time measurement of total organic carbon (TOC) in water. The invention is at least in part based on very rapid, substantially complete mineralization of organics in a water sample by UV irradiation. The irradiated sample is contained in a relatively thin light path sample volume to facilitate rapid mineralization. TOC of the water sample is determined by the detection of the CO2 generated. The microfluidic sample cell of the TOC device of this invention is preferably configured to provide for irradiation of a relatively thin layer of sample to provide very rapid mineralization and to minimize the time required for making a TOC measurement. The method allows direct measurement of TOC of a given sample in a time shorter than about 30 seconds and more preferably provides for sufficiently rapid substantial mineralization (about 2 seconds or less) to allow real-time TOC measurements. The devices of this invention also facilitate minimal lag time between TOC measurement to allow fast, continuous measurements of TOC.
More specifically, this invention provides a device for measurement of TOC in water samples that employs a microfluidic sample cell. As used herein the term microfluidic refers to a sample cell with a sample channel, sample cavity or sample volume having at least one-dimension that is less than about 150 xcexcm. Devices of this invention may employ static or stop-flow sample cells or flow cells. A source of UV radiation is provided for decomposition of organics in a sample within a sample channel, sample cavity or sample volume of the microfluidic sample cell. The sample cell is, at least in part, transparent to UV radiation such that a sample therein can be irradiated to decompose organics to CO2. The sample volume exposed to radiation has a sufficiently thin irradiation path, less than about 150 xcexcm, such that substantially complete mineralization of organics in the irradiated sample volume can occur within about 30 seconds or less, preferably in about 10 seconds or less and more preferably within about 2 seconds or less. The speed of mineralization of a given sample in a microfluidic sample cell will generally also depend upon UV light intensity in the sample volume and the concentration of organics in the sample.
The sample cell of the TOC measurement devices of this invention is optionally provided with a catalyst for enhancing decomposition of CO2 on irradiation. This catalyst can be a photocatalyst, e.g., a photocatalyst that can generate oxidizing agents, such as superoxide and hydroxide radicals, in contact with the sample volume. A preferred photocatalyst is TiO2, and a more preferred photocatalyst is platinized TiO2. Oxygen or other oxidizing agents (e.g., persulfate) can also be introduced into the sample volume to facilitate mineralization of organics.
In the devices of this invention, a UV source is positioned to irradiate a sample volume in the sample cell. If present, the photocatalyst is positioned in contact with the sample volume and positioned to be irradiated by the UV source. Typically, in the device, a UV source is held in a fixed position with respect to the sample cell, for example, in a sample holder. The UV source may be in contact with the sample cell. A reflective surface or mirror can also be provided in the device, e.g., at the bottom of the sample cell, sample channel or sample cavity, to provide for multiple passes of light through the sample volume to increase the optical path length through the sample.
Optionally, an N2 atmosphere or other non-UV-absorbing atmosphere is provided in any space or gap between the UV source and the sample cell. The device is provided with appropriate fluid connections, and/or valves and can be provided with or attached to a fluid pump in order to introduce water samples into or to expel water samples from the sample cell. Alternatively, in on-line configurations, pressure from a water or process line being sampled can be used to introduce and/or expel samples.
TOC measurement devices of this invention are provided with a detector for the CO2 generated on irradiation of organics. A variety of detection methods, including various conductivity methods, infrared and related spectroscopic methods, can be employed. In one embodiment of this invention, CO2 generated by UV irradiation of organics is detected using conductivity measurements. In a second embodiment of this invention CO2 is measured by infrared spectroscopy, preferably nondispersive infrared spectroscopy (NDIR).
In a device of this invention for measurement of TOC using conductivity detection and quantitation of CO2, the sample cell comprises conductivity electrodes in contact with the irradiated sample volume. The sample cell is also provided with a temperature sensing device (preferably in proximity to the conductivity electrodes) or a temperature control device to allow temperature compensation of conductivity measurements. The device is provided with appropriate electrical connections and attendant resistance meters or other output devices for measurement of conductivity and temperature. The temperature of the sample cells, preferably in proximity to the conductivity electrodes, is measured and/or controlled to allow for temperature compensation of conductivity measurements.
In a device of this invention for measurement of TOC using NDIR detection, conductivity electrodes and temperature sensing and/or control are not required. CO2 generated on mineralization is transported or carried to the NDIR detector. The sample can be vaporized in the sample cell by heat generated by UV irradiation or by external heating, alternatively or in addition the sample may be vaporized after mineralization. A heated conduit for transport to the detector is used to avoid condensation on route. Standard NDIR detection methods appropriate for detection of CO2 in the presence of water can be employed.
The TOC devices of this invention can have multiple path sample cells to allow compensation for background conductivity, CO2 or other potential interferants in a non-irradiated control water sample. Multiple path sample cells can also be employed to make comparative TOC measurements of two or more water samples. The surface area of a sample volume exposed to UV irradiation can be varied by selecting the shape of the sample channel with a wider channel providing for a larger exposed surface area than a narrower channel. Similarly, a circuitous or meandering-path channel can be employed to provide for a larger exposed surface area than a simple straight channel. In a flow sample cell configuration, the sample flow rate and the length of channel path irradiated can be varied to adjust the UJV exposure time of a sample passing through the flow cell.
In a device of this invention for measurement of TOC using NDIR spectroscopy for detection and quantitation of CO2, the sample cell is preferably a flow cell.
In one embodiment, the sample cell is formed from two overlaid substrates. A sample channel or cavity for receiving a sample is provided in between the two substrates. This channel or cavity can be provided in one or both of the overlaid substrates (e.g., by etching, micromachining or like process) or it can be provided by a spacer having a cavity or channel intermediate between the two substrates. The substrates can be directly or indirectly bonded together, e.g., using anodic bonding. At least one of the substrates is at least partially transparent to UTV radiation to allow the sample volume to be irradiated. The optional photocatalyst can be provided as a layer in the sample cavity or channel in contact with the sample volume. The photocatalyst is preferably provided as a layer on a substrate surface positioned to be irradiated by the UV light source and in contact with the sample volume.
Water samples can be pretreated, e.g. partially purified, deionized or filtered, before introduction into the microfluidic sample cells of this invention to prevent fouling of the device, minimize background conductivity and/or remove possible interferants. Water samples can, for example, be passed through a filtering device before entering the TOC device to remove particulates and/or passed through an ionic exchange column to reduce background conductivity.
TOC measurement devices of this invention can be used for determination of water quality in supply lines, process lines and the like and can be directly linked on-line for continuous or periodic assessment of TOC. These devices can also be employed in combination with alarm or feedback control systems to detect the presence of an unacceptable TOC level in a water supply line to allow diversion of contaminates and/or prevent contamination of processes. Devices of this invention are particularly useful in applications in the semiconductor industry, particularly with respect to quality of water used in process or cleaning steps.
This invention provides methods for rapid measurement of TOC using the device configurations described hereinabove. Devices and methods of this invention do not rely on prediction of TOC based on partial oxidation of organics in water samples to achieve measurements in less than about 30 sec. However, art-known methods of predicting TOC based on partial oxidation of samples can be coupled to the improved speed of actual measurements provided by the devices of this invention to even further improve the speed of measurements. It is understood that application of such predictive methods can, however, introduce measurement error leading to decreased accuracy of TOC measurements.